1. Field of the Invention
The present invention generally relates to a mobile communication system having a small base station and equipment for its system, and more particularly, to a mobile communication system having a small base station which provides radio communications between the small base station and mobile stations, where the small base station is connected with a central station via optical transmission lines.
For the mobile communication system, in which the base station communicates with such mobile stations as land mobile telephones and portable telephones, a variety of communication systems have been proposed, and several systems are in practical use. For a digital communication system, a time-division multiple access/time-division duplex (TDMA/TDD) communication system is well known. For these mobile communication systems, a large number of base stations of small size and low cost are required.
2. Description of the Prior Art
FIG. 1 shows a block diagram of a conventional mobile communication system. This mobile communication system comprises a central station 201, a base station 202, a downlink transmission line 203.sub.1, an uplink transmission line 203.sub.2, a mobile station such as the land mobile telephone or the portable telephone 204, a combiner 205, a modulator 206, a high-power amplifier 207, a transmit/receive switch 208, a low-noise amplifier 209, a demodulator 210, a divider 211, a controller 212, and an antenna 213.
The central station 201 is connected with the base station 202 through the downlink transmission line 203.sub.1 and the uplink transmission line 203.sub.2 consisting of coaxial cables, etc. A modulated signal at a transmission frequency from the modulator 206 is amplified through the high-power amplifier 207, and the amplified signal is transmitted from the antenna 213 via the transmit/receive switch 208. On the other hand, a received signal at the antenna 213 from the mobile station 204, passes through the transmit/receive switch 208, and is amplified by the low-noise amplifier 209. The amplified signal is then demodulated to a baseband signal by the demodulator 210.
FIG. 2 shows an illustration for explaining a signal frame format of the TDMA/TDD system. One frame (5 ms) consists of four uplink (from the mobile station to the base station) timeslots U1 to U4, and four downlink (from the base station to the mobile station) timeslots D1 to D4, at the same frequency. Each timeslot is allocated to each of subscribers (mobile stations) A, B, C, D, as follows: for example, the uplink timeslot U1 and the downlink timeslot Dl are allocated to the subscriber A, the uplink timeslot U2 and the downlink timeslot D2 are allocated to the subscriber B, the uplink timeslot U3 and the downlink timeslot D3 are allocated to the subscriber C, and the uplink timeslot U4 and the downlink timeslot D4 are allocated to the subscriber D.
The transmit/receive switch 208 in the base station 202 is controlled by the controller 212 so that the antenna 213 is connected with the low-noise amplifier 209 during the uplink timeslot, and the antenna 213 is connected with the high-power amplifier 207 during the downlink timeslot.
As mentioned before, it is hard to miniaturize the conventional base station 202 because many apparatus such as modulator 206 and demodulator 210 must be installed in this base station. Therefore, another mobile communication system having a small base station has been proposed, in which the base station 202 has been miniaturized by installing the modulator 206 and the demodulator 210, etc., into the central station 201, but the base station 202 has been connected with the central station 201 via optical transmission lines.
FIG. 3 shows a block diagram of a typical mobile communication system having a small base station. A configuration that the central station is connected with the small base station via the optical transmission lines, is shown. This system comprises a central station 221, a small base station 222, optical transmission lines 223, and a mobile station 224. The central station 221 further includes a modulator/demodulator (MODEM) 225, a combine/divide section 226, an electric/optic converter section (EO) 227, and optic/electric converter sections (OE) 228, 229. The small base station 222 further includes an optic/electric converter section (OE) 232, a transmit amplifier section (HPA) 233, receive amplifier sections (LNA) 234, 235, electric/optic converter sections (EO) 236, 237, a transmit antenna 238, and receive antennas 239, 240. And a space diversity system is applied for this mobile communication system.
The small base station 222 is constructed with a transmit section 230 consisting of the optic/electric converter section 232 and the transmit amplifier section 233, with a receive section 231 consisting of the electric/optic converter sections 236, 237 and the receive amplifier sections 234, 235. The modulator and demodulator are installed in the central station 221. In this central station 221, a signal is modulated at a different transmission frequency in each modulator/demodulator 225, and each modulated signal is combined in the combine/divide section 226. The combined signal is converted to an optical signal by the electric/optic converter section 227, and the optical signal is transmitted to the small base station 222 via the optical transmission line 223. In the small base station 222, the received optical signal is converted to an electrical signal by the optic/electric converter section 232, and subsequently the electrical signal is amplified through the transmit amplifier section 233, and is transmitted from the transmit antenna 238.
On the other hand, a transmitted signal from the mobile station 224 is received at the antennas 239, 240, and the received signals are respectively amplified through the receive amplifier sections 234, 235. The amplified signals, which indicate signals modulated at the same reception frequency, are converted to optical signals by the electric/optic converter sections 236, 237, and these optical signals are transmitted to the central base station 221 via the optical transmission lines 223. In the central base station 221, the optical signals are converted to electrical signals by the optic/electric converter sections 228, 229, and these electrical signals are divided to each modulator/demodulator 225 by the combine/divider section 226. Each modulator/demodulator 225 demodulates each of the divided signals to a baseband signal. Therefore, such small base station 222 does not need to install the modulator and the demodulator, so that the small base station is suitably applicable for microcell system because of its small size and low cost.
FIG. 4A and FIG. 4B show a graphical representation for explaining transmission and reception performance of an optical signal. FIG. 4A depicts a performance of a semiconductor laser constructing the electric/optic converter section, where a horizontal axis is current I.sub.L, a vertical axis is optical output level P.sub.L, I.sub.th indicates a threshold current, and I.sub.b indicates a bias current. By supplying an operation current having a center value of the bias current I.sub.b, to the semiconductor laser, the optical output in relation to the operation current is obtained. FIG. 4B shows a performance of a photodiode constructing the optic/electric converter section, where a horizontal axis is optical input Pp, a vertical axis is output current Ip. An optical signal is converted to an electrical signal so that the current Ip could flow in relation to an amplitude of the optical input Pp.
As mentioned above, the construction of installing the modulator and the demodulator at the central station, and connecting between the central station and the small base station via optical transmission lines, enables miniaturization of the base station. In such system, an interface between the central station and the small base station is defined. For instance, an output level of the combine/divide section 226 in the central station 221 needs to be same as an input level of the transmit amplifier section 233 in the small base station 222, and an output level of the receive amplifier sections 234, 235 in the small base station 222 needs to be same as an input level of the combine/divide section 226 in the central station 221.
In this definition, the output level Pa of the combine/divide section 226 is described as Pa=n.times.P.sub.0, where n is a number of carriers, and a carrier level is P.sub.0. The combine/divide section 226 combines carriers corresponding to the modulator/demodulator 225 installed in the central station 221. However, an actual number of carriers which are combined in the combine/divide section 226 is varied according to activation of each modulator/demodulator 225 by call generation and call termination, so that the output level of the combine/divide section 226 is also varied. And a maximum transmission power of an optical fiber constructing the optical transmission lines 223, is limited by nonlinear phenomena such as induced Raman dispersion, induced Brillouin dispersion, self-phase modulation, and four-wave parametric combining. Further, a maximum transmission power of a semiconductor laser constructing the electric/optic converter section is also limited due to its structure.
Therefore, in the conventional system, an automatic-output control function is generally implemented, whereby the semiconductor laser constructing the electric/optic converter section is controlled to detect the optical output and to feedback the detected value to a driving-current source so as to maintain the optical output constant. Therefore, under this situation the transmit amplifier section 233 in the small base station 222 always receives a constant signal level independent of the number n of the carriers to be combined, and amplifies the signal by a constant gain. Thus, a transmission signal having a constant power level is always transmitted from the small base station 222 regardless of the number of carriers to be combined. In this condition, even if the number n of carriers to be combined increases, a proper transmission level which is proportional to the number n of carriers can not be obtained, so that a problem occurs, in which a receiving signal level at the mobile station is reduced.
On the other hand, as shown in FIG. 3, the small base station 222 includes one transmit antenna 238 and two receive antennas 239, 240. By using one antenna in common for transmitting and receiving as shown in FIG. 1, it enables further miniaturization of the base station. However, when applying the TDMA/TDD system to this configuration, there is a need for transmit/receive switching between the transmit timeslot and the receive timeslot for the mobile station. Furthermore, if applying a circulator as a duplex for the transmit/receive switch, a problem occurs, in which a transmission signal turns into the receive section, and a reception signal turns into the transmit section, because a transmission frequency is the same as a reception frequency.